1. Field of the Invention
The present invention relates to an improved apparatus and process for blow molding, particularly to an apparatus and process for producing a hollow double wall type plastic article by the blow molding.
2. Description of the Related Art
The blow molding process is a well known method of producing hollow plastic articles. Initially, the blow molding process was directed to the production of a hollow article having a solid wall with a generally convex peripheral outer surface. Most of such hollow articles are basically rotation symmetrical or plane symmetrical. Recently, a modified blow molding process of producing a hollow article having a basically concave, convex or bowl shape with a hollow double wall, has been proposed.
This modified blow molding process uses a mold consisting of a male or convex mold half and a female or concave mold half, both mold halves defining a mold cavity having a concave shape. The hollow double wall type article has a concave inner surface portion and a convex outer surface portion, both forming the hollow double wall with a substantially uniform small gap between the two surface portions over substantially the entire area thereof. Most of such hollow double wall type articles are designed to have an overall contour showing relatively shape angled corners, since they are inherently intended for use in place of conventional solid wall type articles produced by a normal injection molding method or a vacuum molding method.
In this connection, the modified blow molding process is intended to effect a formation of such convex and concave surface portions, each having a substantially uniform resin thickness over the entire surface area, as far as possible, but it has been realized that the resin thickness at the corners is likely to be smaller than that at the other parts of the hollow double wall, due to the nature of the blow molding. Further, it is noted that a mold for use in the modified blow molding has a larger cavity surface with a smaller cavity volume or space, than those of the mold for use in the original blow molding. Therefore, a considerably higher molding capability is required for the modified blow molding than that for the original blow molding.
To cope with this inherent problem involved in the production of a hollow double wall type article, various types of improved heating and cooling systems to be incorporated in a mold have been provided.
For example in a prior art blow molding apparatus as shown in FIG. 6, the mold has male and female mold halves 3 and 4, both made entirely of a metallic material. Each mold half 3 and 4 has spaced pipes 5 embedded, or has spaced through-holes formed, in the metallic body in the vicinity of a cavity surface 3a or 4a. The pipes are used for heating the cavity surface, which has been cooled in the last molding cycle, with a heating medium such as hot water to a temperature of 20.degree. C. to 80.degree. C., necessary before a mold cavity can receive a resin portion 2 of a hot plasticized resin extruded from an extruder 1.
The heating of the cavity surfaces is intended to promote the function of the cavity surfaces, in cooperation with the hot resin portion forced to expand by blowing air thereinto using an air injector, to cause the expanding resin portion, as a whole, to be effectively deformed along the cavity surfaces and to fit thereon. The same pipes 5 are used in the same molding cycle to cool the cavity surface of the mold by flowing a cooling medium therethrough just before the blow molding is completed, to thereby cool a molded article in the cavity. These heating and cooling steps are carried out in each molding cycle.
In other prior art disclosed in JP B 63-2769, intended for a production not of a hollow double wall type article but of a normal hollow article such as a plastic bottle, although it may be applied for a production of such a hollow double wall type article, spaced passages formed in mold halves made entirely of a metallic material are similar to those of the first mentioned prior art but are different therefrom in that a laminate film consisting of an electric insulator film and an electric conductor film coated thereon, are used which laminate film is attached to a corresponding cavity surface of each mold half over the entire area thereof. Heating is effected by flowing a current through the attached film, and cooling is effected by flowing a cooling medium through the passages.
In other prior art as disclosed in JP A 59-54524 is directed to the blow molding apparatus as shown in FIG. 7. A male mold half 3 made entirely of a metallic material has spaced passages 5 formed therein at corners of a cavity surface 3a, through which a cooling medium flows, and electric heating coil units 6 embedded therein near the corners, and a female mold half 4 made entirely of a metallic material is provided with the same heating coil units 6 embedded therein near corners of a cavity surface 4a, and with two groups of perforated disks 7 and 8 dispersed therein at the corners, the first group of disks 7 being associated with passages formed in the mold half communicating with a cold gas source and the second group of disks 8 being associated with other passages 10 formed in the mold half for discharging the cold gas fed from the first group disks 7 into a mold cavity. Alternatively, the cooling passages 5 of the male mold half 3 may be replaced by the some first and second group of disks 7 and 8 and passages 9 and 10 associated therewith as those of the female mold half 4, or the first and second group of disks 7 and 8 of the female mold half 4 may be replaced by the same cooling passages 5 as those of the male mold half 3. The heating coil units 6 are actuated to heat the cavity surfaces before a resin portion 2 of a hot plasticized resin extruded from an extruder 1 is introduced into a mold cavity. While the resin portion is in the process of expansion in the cavity, due to a gas blowing into the resin portion from a gas injector or a blow gas pin, a cold gas or liquid medium is forced to flow through the cooling passages 5 of the male mold half 3 to cool the corner parts of the cavity surface, and a cold gas medium is forced to flow into the cavity from the first group of disks 7 and is discharged from the second group of disks 8 to thereby cool parts of the resin portion to be molded to form corner parts of a resultant article.
The heating and cooling system is intended to effectively prevent a blow molded article from having corner parts with a thinner thickness relative to the other parts, whereby the molded article of a hollow double wall type is provided with a resin thickness with less variation over the entire surface.
JP A 59-54524 does not disclose a cooling of the molded article in the mold cavity just before the mold is opened, but it is assumed, as a matter of nature, that the molded article would be cooled by using the cooling passages 5. Nevertheless a question remains of how effectively the passages 5, originally intended for use in cooling the corner parts of the cavity surfaces, work to cool the mold articles as a whole.
The above-mentioned prior art devices are able to attain some improvements for higher quality articles, particularly hollow double wall type articles having a resin thickness with less variations, but the present inventors realized that they share the some serious problem in that the adopted heating and cooling system incorporated in the mold causes a blow molding cycle to take a considerably longer time than that of the above mentioned original blow molding for producing solid wall type hollow articles. This is because the original blow molding process does not always use both heating and cooling steps, but may have only a cooling step for cooling a molded article before a removal thereof from a mold. In this connection, the modified blow molding process always requires the time necessary for the heating step, in addition to the time for the cooling step, at every molding cycle.
Further, it is most important to note that the modified blow molding process requires alternately and repeatedly heating and cooling the mold, while the molding cycle is repeated.
The present inventors recognized that the above repetition causes a substantial operational delay due to the alternate cooling and heating, with the result that the molding cycle time is prolonged by the operational delay.
Still further, it is noted that the conventional modified blow molding processes have another problem in that the cavity surfaces of the mold are not cooled in such a way that a uniform cooling effect for the molded article is exhibited over the entire area thereof, i.e., they are cooled to exhibit a variation of cooling effect over the entire area.
Such a variation of the cooling effect is likely to lead to molded articles having poor qualities such as poor dimensional accuracy and surface shrinkage, and/or a surface quality poor in gloss or poor in image transfer or transcription from the contoured cavity surface.